Anthropogenic Organic Compounds in Source Water of Selected Community Water Systems that Use Groundwater, 2002–05

By Jessica A. Hopple, Gregory C. Delzer, and James A. Kingsbury

Abstract

Source water, defined as groundwater collected from a
community water system well prior to water treatment, was
sampled from 221 wells during October 2002 to July 2005
and analyzed for 258 anthropogenic organic compounds. Most
of these compounds are unregulated in drinking water and
include pesticides and pesticide degradates, gasoline hydrocarbons,
personal-care and domestic-use products, and solvents.
The laboratory analytical methods used in the study have
detection levels that commonly are 100 to 1,000 times lower
than State and Federal standards and guidelines for protecting
water quality. Detections of anthropogenic organic compounds
do not necessarily indicate a concern to human health but
rather help to identify emerging issues and track changes in
occurrence and concentrations over time.

Less than one-half (120) of the 258 compounds were
detected in at least one source-water sample. Chloroform, in
36 percent of samples, was the most commonly detected of
the 12 compounds that were in about 10 percent or more of
source-water samples. The herbicides atrazine, metolachlor,
prometon, and simazine also were among the commonly
detected compounds. The commonly detected degradates of
atrazine—deethylatrazine and deisopropylatrazine—as well as
degradates of acetochlor and alachlor, generally were detected
at concentrations similar to or greater than concentrations
of the parent herbicide. The compounds perchloroethene,
trichloroethene, 1,1,1-trichloroethane, methyl tert-butyl ether,
and cis-1,2-dichloroethene also were detected commonly. The
most commonly detected compounds in source-water samples
generally were among those detected commonly across the
country and reported in previous studies by the U.S. Geological
Survey’s National Water-Quality Assessment Program.

Relatively few compounds were detected at concentrations
greater than human-health benchmarks, and 84 percent of
the concentrations were two or more orders of magnitude less
than benchmarks. Five compounds (perchloroethene, trichloroethene,
1,2-dibromoethane, acrylonitrile, and dieldrin) were
detected at concentrations greater than their human-health
benchmark. The human-health benchmarks used for comparison
were U.S. Environmental Protection Agency Maximum Contaminant Levels (MCLs) for regulated compounds
and Health-Based Screening Levels developed by the U.S.
Geological Survey in collaboration with the U.S. Environmental
Protection Agency and other agencies for unregulated
compounds. About one-half of all detected compounds do not
have human-health benchmarks or adequate toxicity information
to evaluate results in a human-health context.

Ninety-four source-water and finished-water (water that
has passed through all the treatment processes but prior to
distribution) sites were sampled at selected community water
systems during June 2004 to September 2005. Most of the
samples were analyzed for compounds that were detected
commonly or at relatively high concentrations during the
initial source-water sampling. The majority of the finished-water
samples represented water blended with water from
one or more other wells. Thirty-four samples were from water
systems that did not blend water from sampled wells with
water from other wells prior to distribution.

The comparison of source- and finished-water samples
represents an initial assessment of whether compounds present
in source water also are present in finished water and is not
intended as an evaluation of water-treatment efficacy. The
treatment used at the majority of the community water systems
sampled is disinfection, which, in general, is not designed to
remove the compounds monitored in this study.

Concentrations of all compounds detected in finished
water were less than their human-health benchmarks. Two
detections of perchloroethene and one detection of trichloroethene
in finished water had concentrations within an order
of magnitude of the MCL. Concentrations of disinfection
by-products were expected to increase in finished water relative
to source water because of disinfection. The MCL for
concentrations of disinfection by-products analyzed in this
study is for total trihalomethanes, and concentrations were
within an order of magnitude of the MCL in finished water
from nine sites, but only three detections of chloroform and
one detection of bromoform were within an order of magnitude
of the MCL. Concentrations of all other compounds
were more than an order of magnitude less than human-health
benchmarks.

About one-half (57 percent) of the detections from the
34 community water systems where finished water was not
blended with other source water were in both source and
finished water, and concentrations were similar, with the
exception of disinfection by-products. Most of the detections
were gasoline-related compounds, herbicides and herbicide
degradates, and solvents. Results for blended finished-water
samples generally were similar to nonblended finished-water
samples, and blending did not always reduce concentrations
to less than the laboratory reporting level. Differences in the
occurrence of compounds in source and finished water could
be the result of water treatment, volatilization, blending, or
analytical variability at concentrations near or less than the
laboratory reporting level. Large changes in concentration
from source to finished water of solvents in a few samples
likely are attributable to additional water treatment steps used
by the CWS to treat water known to contain elevated concentrations
of organic compounds.

Mixtures of two or more compounds were detected in
about 70 percent of source-water samples and in 82 percent
of finished-water samples. Mixtures occur more commonly
in finished water than source water because of the formation
of disinfection by-products in finished water. The potential
human-health significance of the frequent presence of
mixtures of organic compounds in finished water remains
largely unknown.